Low power balanced colpitts oscillator with improved negative resistance

ABSTRACT

A low power balanced Colpitts oscillator circuit with improved negative resistance. The oscillator circuit comprises two cross-coupled Colpitts oscillators with a crystal oscillator connected between the two oscillators. A single current source can be used for both Colpitts oscillators since only one Colpitts needs a current source at a time. Using a single current source cuts the power consumption in half. Alternatively, a transistor in each Colpitts oscillator can act as a current source, which is turned on or off depending on the state of the Colpitts transistor. The two current sources are biased at a common level and matched to ensure the circuit remains symmetrical. An additional benefit of the present invention is that the negative resistance is directly improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oscillator circuit. Morespecifically, the present invention discloses a low power balancedColpitts oscillator with improved negative resistance.

2. Description of the Prior Art

Oscillator circuits are used for providing a reference frequency forother circuits or components in a system. As frequency requirementscontinue to rise, stability and phase noise performance become issuesthat can limit the effectiveness or efficiency of the oscillatorcircuit. This in turn directly affects the other circuitry utilizing thefrequency signal.

In applications where phase noise and jitter sensitivity is of concern,standard phase locked loops are typically not usable. In theseapplications, non-PLL circuits are required.

Additionally, conventional oscillators can consume more power than isideal for a low power consumption device.

Therefore, there is need for an efficient and effective oscillatorcircuit with low power consumption and improved negative resistance.

SUMMARY OF THE INVENTION

To achieve these and other advantages and in order to overcome thedisadvantages of the conventional method in accordance with the purposeof the invention as embodied and broadly described herein, the presentinvention provides a low power balanced Colpitts oscillator withimproved negative resistance.

The low power balanced oscillator circuit comprises a crystal oscillatorconnecting two cross-coupled Colpitts oscillators. The oscillators arecross-coupled by either a direct connection or by capacitors. Thecrystal oscillator provides the Colpitts oscillators with a referencefrequency that is half a duty cycle out of phase for each Colpittsoscillator. In this way, the oscillator circuit effectively doubles thereference frequency.

A single current source can be used for both Colpitts oscillators. Incontrast, if two current sources were used they would both be constantlyactive even though it isn't necessary whenever their respective Colpittstransistor is turned on. When one Colpitts transistor is on, the currentsource connected to the opposite transistor could in effect be turnedoff. Additionally, a transistor in each Colpitts oscillator can act as aswitch that is controlled by the opposite Colpitts transistor's gatevoltage due to cross-coupling the two Colpitts oscillators. Thereforeefficient operation can be achieved by using a single current source,which reduces the current and power consumption in half.

Alternatively, a transistor in each Colpitts oscillator can be biasedand act as a current source. These current sources are turned on or offdepending on the state of the Colpitts transistor.

Furthermore, the sources of the transistors acting as current sourcescan be connected to ground. Grounding this node further improves thesmall loop gain and directly improves the absolute value of the negativeresistance.

As a result, the present invention provides an efficient and effectiveoscillator with low power consumption and improved negative impedance.

These and other objectives of the present invention will become obviousto those of ordinary skill in the art after reading the followingdetailed description of preferred embodiments.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

FIG. 1 is a circuit schematic of low power balanced Colpitts oscillatoraccording to an embodiment of the present invention; and

FIG. 2 is a circuit schematic of low power balanced Colpitts oscillatoraccording to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

Refer to FIG. 1, which is a circuit schematic of low power balancedColpitts oscillator according to an embodiment of the present invention.

As shown in FIG. 1, the low power balanced Colpitts oscillator 100comprises a crystal oscillator 105 electrically connected between thegates of a first transistor 110 and a third transistor 115. A firstcapacitor 130 is connected between the gate and the source of the firsttransistor 110. A second capacitor 140 is connected between the sourceof the first transistor 110 and ground. The drain of a second transistor120 is connected to the source of the first transistor 110.

A third capacitor 135 is connected between the gate and the source ofthe third transistor 115. A fourth capacitor 145 is connected betweenthe source of the third transistor 115 and ground. The drain of a fourthtransistor 125 is connected to the source of the third transistor 115.The sources of the second transistor 120 and the fourth transistor 125are connected to a current source 150.

Furthermore, the gates of the first transistor 110 and the fourthtransistor 125 are connected to each other and the gates of the secondtransistor 120 and the third transistor 115 are connected to each other.

A filter 170, comprising an inductor 180 and a filter capacitor 190connected in parallel, is connected to the drains of the firsttransistor 110 and the third transistor 115. The filter 170 filters outthe DC component present and further attenuates higher frequencyharmonics.

A coupling capacitor 160 couples the oscillator circuit 100 to the nextstage through AC coupling.

The Balanced Colpitts oscillator of the present invention basicallycomprises two oscillator circuits configured in a cross-coupled balancedconfiguration. Additionally, a crystal oscillator is connected betweenthe gates of the transistors of both oscillator circuits.

The crystal oscillator provides a reference frequency to theoscillators. The reference frequency supplied to the first oscillator isa half period out of phase with the frequency supplied to the secondoscillator. The drains of the transistors of each oscillator areconnected together and this node will see the sum of the currentsthrough each oscillator. As a result, the circuit of the presentinvention effectively doubles the reference frequency of the crystaloscillator.

In operation, the second transistor 120 and the fourth transistor 125are controlled by the gate voltage of the third transistor 115 and thefirst transistor 110 respectively. In this way, the transistors are in across-coupled manner.

Taking advantage of the synchronized turning-on and off of the secondtransistor 120 and the fourth transistor 125, only one shared currentsource 150 is needed and is connected to both the second transistor 210and the fourth transistor 125. As a result, the desired operation isachieved by using a single current source 150 instead of two. Therefore,the current and power consumption is reduced in half.

An additional benefit from the cross-coupling is the enhancement ofnegative resistance by 20 to 30% of the resulting circuit. This is verydesirable in high frequency crystal oscillators and voltage controlledcrystal oscillators.

Refer to FIG. 2, which is a circuit schematic of low power balancedColpitts oscillator according to an embodiment of the present invention.

As shown in FIG. 2, the balanced Colpitts oscillator 200 comprises acrystal oscillator 205 electrically connected between the gates of afirst transistor 210 and a third transistor 215. A first capacitor 230is connected between the gate and the source of the first transistor210. A second capacitor 240 is connected between the source of the firsttransistor 210 and ground. The drain of a second transistor 220 isconnected to the source of the first transistor 210.

A third capacitor 235 is connected between the gate and the source ofthe third transistor 215. A fourth capacitor 245 is connected betweenthe source of the third transistor 215 and ground. The drain of a fourthtransistor 225 is connected to the source of the third transistor 215.The sources of the second transistor 220 and the fourth transistor 225are connected to ground.

A fifth capacitor 250 is connected between the gate of the thirdtransistor 215 and the gate of the second transistor 220. A sixthcapacitor is connected between the gate of the first transistor 210 andthe gate of the fourth transistor 225.

In order to maintain symmetry in the circuit, the gates of the secondtransistor 220 and the fourth transistor 225 are connected to a biasnode 299 via a first resistor 260 and a second resistor 265respectively. A seventh capacitor 270 and eighth capacitor 275 are alsoconnected to the gates of the second transistor 220 and the fourthtransistor 225 respectively. The other ends of the seventh capacitor 270and eighth capacitor 275 are floating.

A filter 285, comprising an inductor 290 and a filter capacitor 295connected in parallel, is connected to the drains of the firsttransistor 210 and the third transistor 215. The filter 285 filters outthe DC component present and further attenuates higher frequencyharmonics.

A coupling capacitor 280 couples the oscillator circuit 200 to the nextstage through AC coupling.

Similar to the operation described in regards to FIG. 1, the low-powerBalanced Colpitts oscillator of the present invention basicallycomprises two oscillator circuits configured in a cross-coupled balancedconfiguration with a crystal oscillator connected between the gates ofthe transistors of both oscillator circuits. The crystal oscillatorprovides a reference frequency to the oscillators, which is supplied tothe first oscillator a half period out of phase with the frequencysupplied to the second oscillator. The drains of the transistors of eachoscillator are connected together and this node will see the sum of thecurrents through each oscillator. As a result, the circuit of thepresent invention effectively doubles the reference frequency of thecrystal oscillator.

In this embodiment, the second transistor 220 and the fourth transistor225 are turned on or off depending on the state of the third transistor215 and first transistor 210 respectively. In order to ensure that thecircuit remains symmetrical, the second transistor 220 and fourthtransistor 225 are biased at a common level and matched.

An advantage of the circuit illustrated in FIG. 2, is that by connectingthe sources of the second transistor 220 and the fourth transistor 225to ground, the small loop gain is improved. Therefore, the absolutevalue of the negative resistance directly increases. As a result a 100%to 200% improvement of the negative impedance can be achieved.

Therefore, the present invention provides a low power balanced Colpittsoscillator with improved negative resistance. By utilizing thesynchronized turning-on and off of the switches, only one shared currentsource is required. As a result, the current and power consumption isreduced by half. Additionally, due to the cross-coupling, the circuit ofthe present invention has enhanced negative resistance, making thecircuit ideal for high frequency crystal oscillator and voltagecontrolled crystal oscillator applications.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the scope or spirit of the invention. In view ofthe foregoing, it is intended that the present invention covermodifications and variations of this invention provided they fall withinthe scope of the invention and its equivalent.

1. An oscillator circuit comprising: a first Colpitts oscillator; asecond Colpitts oscillator cross-coupled to the first Colpittsoscillator; and a crystal oscillator connected between the firstColpitts oscillator and the second Colpitts oscillator.
 2. Theoscillator circuit of claim 1, the first Colpitts oscillator comprising:a first transistor comprising a first gate, a first source, and a firstdrain; a first capacitor connected between the first gate and the firstsource of the first transistor; a second transistor comprising a secondgate, a second source, and a second drain, the second drain connected tothe first source; and a second capacitor connected between the firstsource and ground.
 3. The oscillator circuit of claim 2, the secondColpitts oscillator comprising: a third transistor comprising a thirdgate, a third source, and a third drain; a third capacitor connectedbetween the third gate and the third source of the third transistor; afourth transistor comprising a fourth gate, a fourth source, and afourth drain, the fourth drain connected to the third source and thefourth source connected to the second source; and a fourth capacitorconnected between the third source and ground.
 4. The oscillator circuitof claim 3, further comprising: a filter connected to the first drain ofthe first transistor and the third drain of the third transistor.
 5. Theoscillator circuit of claim 1, further comprising: a coupling capacitorto couple the oscillator circuit to a next stage.
 6. The oscillatorcircuit of claim 3, further comprising: a current source connected tothe second source and the fourth source.
 7. The oscillator circuit ofclaim 3, the cross-coupling comprising electrically connecting the thirdgate and the second gate and electrically connecting the first gate andthe fourth gate.
 8. The oscillator circuit of claim 3, furthercomprising: a fifth capacitor connected between the third gate and thesecond gate; and a sixth capacitor connected between the first gate andthe fourth gate.
 9. The oscillator circuit of claim 8, furthercomprising: a bias node; a first resistor connected between the secondgate and the bias node; and a second resistor connected between thefourth gate and the bias node.
 10. A low power balanced oscillatorcomprising: a first transistor comprising a first gate, a first source,and a first drain; a second transistor comprising a second gate, asecond source, and a second drain, the second drain connected to thefirst source; a third transistor comprising a third gate, a thirdsource, and a third drain, the third drain connected to the first drain;a fourth transistor comprising a fourth gate, a fourth source, and afourth drain, the fourth drain connected to the third source and thefourth source connected to the second source; a crystal oscillatorelectrically connected between the first gate of the first transistorand the third gate of the third transistor; a first capacitor connectedbetween the first gate and the first source of the first transistor; asecond capacitor connected between the first source and ground; a thirdcapacitor connected between the third gate and the third source of thethird transistor; and a fourth capacitor connected between the thirdsource and ground.
 11. The low power balanced oscillator of claim 10,further comprising: a current source connected to the second source andfourth source.
 12. The low power balanced oscillator of claim 10,wherein the first gate is connected to the fourth gate and the secondgate is connected to the third gate.
 13. The low power balancedoscillator of claim 10, further comprising: a fifth capacitor connectedbetween the first gate and the fourth gate; and a sixth capacitorconnected between the second gate and the third gate.
 14. The low powerbalanced oscillator of claim 13, further comprising: a bias node forbiasing the second and fourth transistors; a first resistor connectedbetween the second gate and the bias node; and a second resistorconnected between the fourth gate and the bias node.
 15. The low powerbalanced oscillator of claim 10, further comprising: a filter connectedto the first drain of the first transistor and the second drain of thesecond transistor; and a coupling capacitor to couple the low powerbalanced oscillator to a next stage.
 16. A low power balanced oscillatorcomprising: a first transistor comprising a first gate, a first source,and a first drain; a second transistor comprising a second gate, asecond source, and a second drain, the second drain connected to thefirst source; a third transistor comprising a third gate, a thirdsource, and a third drain, the third drain connected to the first drain;a fourth transistor comprising a fourth gate, a fourth source, and afourth drain, the fourth drain connected to the third source and thefourth source connected to the second source; a crystal oscillatorelectrically connected between the first gate of the first transistorand the third gate of the third transistor; a first capacitor connectedbetween the first gate and the first source of the first transistor; asecond capacitor connected between the first source and ground; a thirdcapacitor connected between the third gate and the third source of thethird transistor; a fourth capacitor connected between the third sourceand ground; a filter connected to the first drain of the firsttransistor and the second drain of the second transistor; and a couplingcapacitor to couple the low power balanced oscillator to a next stage.17. The low power balanced oscillator of claim 16, further comprising: acurrent source connected to the second source and fourth source.
 18. Thelow power balanced oscillator of claim 16, wherein the first gate isconnected to the fourth gate and the second gate is connected to thethird gate.
 19. The low power balanced oscillator of claim 16, furthercomprising: a fifth capacitor connected between the first gate and thefourth gate; and a sixth capacitor connected between the second gate andthe third gate.
 20. The low power balanced oscillator of claim 19,further comprising: a bias node for biasing the second and fourthtransistors; a first resistor connected between the second gate and thebias node; and a second resistor connected between the fourth gate andthe bias node.